I think you've got yourself confused. The heap is what malloc is allocating. So it's like you said "Surely this is somewhere in Europe, not France?" or "Surely this is a round shape, not a circle?"

Not when we’re talking about a JVM, which has its own heap that does not use malloc.

The JVM is of course just software. In this case it's mostly software written in C++ and that software allocates "its own heap" using uh... malloc

https://github.com/openjdk/jdk/blob/master/src/hotspot/share...

In the specific case of the OpenJDK, there isn't the one JVM, rather a standard[0], like in ISO languages, with a reference implementation to go along it,

https://en.wikipedia.org/wiki/List_of_Java_virtual_machines

And the list doesn't include all, e.g. PTC (https://www.ptc.com/en/products/developer-tools/perc)

[0] - https://docs.oracle.com/javase/specs/index.html

malloc is also not a specific implementation, but a standardized interface for allocating on the heap.

I'm actually pretty surprised if the JVM uses malloc to allocate the heap. V8 uses mmap, never malloc.